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Crunching Numbers

OK - I just started up the number cruncher. Let's see what we have here. What the heck's the bore of this engine anyway? I'll use 66.5 mm's to keep it legal in the AMA's 250 class. OK, so we have a 263 cc engine with a long rod. At 9000 rpm the piston is going 4488 FPM. That's kind of high for sustained operation, but not weird. For a motocross engine the numbers don't make sense for a 250 sized engine. At 8050 rpm's the engine is at 4014 FPM - that's a long way from 9000.

I think for motocross you'll find that 180 to 182 degrees of exhaust is good. With a decent compression ratio it would pull like a Detroit diesel, have a fair to moderate hit on the right pipe and rev to about 7500 with usable overrev to 8000 or so. Usually, motocross riders need tractable power with few hits in the power band though they do need explosive power for the short straights between the jumps. Transfer port duration of about 120 - 122 degrees will allow it to fill well enough. The key here is scavenging efficiency, velocity through the ports and everything in moderation. If the exhaust port gets more duration, the engine becomes more peaky - taken further it becomes poorly matched and will carburet poorly when on the pipe. Raise the transfer port duration and the power band shifts upward a bit, but comes into tune a little better with the longer exhaust duration. Raise the transfer duration higher and the engine becomes sluggish - won't like to rev due to lack of blowdown but tends to favor upper rpm's even if it doesn't seem to make good power there.

There are a thousand combinations to work with when designing engines. When speaking of engines with fixed exhaust port heights the choices are tough. The power valved 250's have port timing from mild to wild. They chug down low and rev out on top. It's accomplished by using the engines displacement/long power stroke to move the bike at low revs, and its rev ability to make the power for high speeds. The pipe is a compromise though, favoring the upper mid range. Again, engine displacement and long power stroke down low to correct the pipes/ports mistakes and lots of shorter power pulses up top to help fix its shortcomings there. The sweet spot of the engine - where everything comes into tune is in the neighborhood of 4565 to 7680, with good overrev to about 9000. The engines peak torque target is at 6685 rpm.

Engines favoring more aggressive riding, like dune riders/racers tend to be built with more port timing. Numbers like 188 to 192 degrees of exhaust and 126 to 128 degrees of transfer port duration. A compression ratio of 15:1 to 17.5:1 seems to work well - depending on the octane chosen. Top end revs suffer somewhat due to elevated compression ratios but in the dunes it seems to work real well.

Taking this a little further and into a more drag style arrangement the numbers change to as much as 194 to 202 degrees of exhaust duration and 128 to 132 degrees of transfer port duration. For elevated revs the blowdown needs to be increased or the peak power will suffer tremendously. Again the compression ratio is decided upon by octane and so on, but these specially built machines become so purposefully built that the tiniest change in combustion chamber volume ruins the targeted speed for the targeted race distance. These engines if built for 300 ft drag don't do as well on longer 600 ft or 800 ft races unless geared very carefully and then they're still somewhat out of their environment.

Building a stroker isn't that tough. What's tough is deciding on its intended target. Most of the guys I build engines for are recreational riders so they want an engine to take them through the woods as well as win every race when they go to the dunes. That's why I run across so many mis-matched engine combinations. Sometimes a guy buys a rev pipe when he actually needs more displacement. Or adds a big carburetor when he really needs to increase the compression ratio. Again there a thousand combinations. Only about 10% of the people I've built engines for in the past twelve months actually race sanctioned events. For those that race their target is very clear. They don't need their engines to do everything. They specify the track type and length and often can describe exactly the kind of power band they need to match their riding style. This is why communication between the engine builder and rider becomes so important. That's why I'm often on the phone until 10:00 PM or 11:00 PM and sometimes talk all day with riders and get nothing done in the shop. It is so important for the engine builder to communicate well with the rider and it's so important for the rider to communicate with the engine builder.

My kids think I speak a foreign language. They walk around saying things like "Dad's got a short rod...heh. What the heck is up with that?!? A stroke? Is he sick? Dad, are Honda's better?" How funny! I have to be careful how I answer that one because my son has a Blaster and my daughter has an LT80. If I go around saying Honda is the best I'm likely to end up having to buy a couple more Honda's. So instead I tell them about what I rode when I was a kid. After a few minutes of that they're so bored they forgot what they asked. You know, to kids a parents past experiences never really happened. All they know is what I had sucked and I was happy to have one that did so.

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Crunching Numbers